The invention relates to the field of optically confining structures, and in particular to mode transformers for low index high confinement waveguides.
Strong light confining, low loss propagating, and converting are the three fundamental issues for a miniaturized, high speed, and integrated light-wave circuit. The conventional light waveguiding approach is based on optical waveguides made of a high-index core surrounded by low-index claddings. The total internal reflection (TIR) at the core/cladding boundaries ensures most of the energy confined inside the core regions. High index contrast waveguides, such as silicon strip waveguides, have the benefit of strong confinement and low loss which satisfy the requirement of the planar photonic integration. However, the problems associated with mode conversion between high index contrast waveguides and low index contrast waveguides still remain.
In some other circumstances, such as in the photonic bandgap (PBG) structures and anti-resonant reflecting optical waveguides (ARROWs), by utilizing the external reflection induced by the multiple-dielectric-layer interferences, light can be confined and guided in the low-index core. To confine light inside the low index core regions, the resonant condition requires the core dimensions to be on the order of the wavelength.
Very recently, a novel structure named slot waveguide has been proposed and demonstrated the capability of guiding and confining light in low-index media in the nanometer size range. The structure consists of a single or multiple thin low-index slots (layers) embedded between high-index regions. Due to the large index contrast at interfaces, the normal electric field undergoes a large discontinuity, which results in a high field concentration in the low-index regions. Depending on applications, the structure can be vertical slots and horizontal layers, nevertheless, they are considered low index high confinement waveguides.
High index contrast waveguides, ARROR/PBG waveguides, and low index high confinement slot waveguides not only have different geometries but also show different physical properties. Geometrically, ARROW/PBG waveguides have the largest dimensions and mode sizes, therefore are suitable for fiber coupling. Slot waveguides, although of comparable dimensions and mode sizes to the high index contrast strip waveguides, have a large mode mismatching between them are quite big, which is indicated by the effective index difference. Obviously, low-loss mode transformers between waveguides are highly desired in order to realize multi-functional integrated photonic devices and circuits.